(1) Field of the Invention
This invention relates to an image display apparatus capable of reproducing tone by utilizing a picture element pattern composed of a plurality of dots as a unit.
(2) Description of the Prior Art
Heretofore a method for tone reproduction wherein a plurality of dots are used as a unit, and such picture element pattern in which display or recorded density of the respective dots is controlled gradually in response to density of its original image (including any gradation which has not been recorded) is utilized has been known as one of tone reproduction methods. Such method is one wherein when a part of image having a certain tone density is recorded, such a pattern determined unequivocally in response to the tone density is repeatedly expressed or recorded to reproduce the tone of the above part.
As an example, a tone reproduction method to which a 3-level Dither method is applied will be described hereinbelow. Now, explanation will be made of such 3-level Dither method wherein a density level of three gradations of 0 (means no recording), 1/2, and 1 can be displayed or recorded. In this 3-level Dither method, when density of attention read picture element resides in coordinate position (m, n) of the image in an original, the displaying or recording level of the picture element, the threshold value between recorded density levels of 0 and 1/2, and the threshold value between recorded density levels of 1/2 and 1 are I (m, n), P (m, n), a (m, n), and b (m, n), respectively. Relationships between or among them may be expressed as follows.
(1) P (m, n)=0 in case of I (m, n)&lt;a (m, n) PA0 (2) P (m, n)=1/2 in case of a (m, n).ltoreq.I (m, n)&lt;b (m, n) PA0 (3) P (m, n)=1 in case of b (m, n).ltoreq.I (m, n)
According to the above 3-level Dither method, when threshold values a (m, n) and b (m, n) are suitably changed, contrast and density of the image recorded can be changed.
FIG. 1 illustrates an example of picture element pattern wherein as area of a certain tone density is expressed by 3-level Dither method, 2.times.2 matrix, and FIG. 2 shows a reproduced image of continuous area with the same tone density expressed by utilizing the picture element pattern of FIG. 1.
As is apparent from FIG. 2, the tone reproduced image is composed of vertical lines A and B of two different densities. More specifically, one line A is a vertical line expressed by continuous dots of 1 level density, while the other line B is another vertical line obtained by disposing alternately two types of dots of 0 level and 1/2 level.
A tone expressed by such two kinds of vertical lines of different density is visually recognized as a striped pattern obtained by repeating said two kinds of lines A and B. Namely, the method as mentioned above involves such problem that a reproduced image having a wide area of uniform tone density cannot be obtained.
Such case of a reproducing image having a wide area of the same tone density is not only accompanied by the problem as mentioned above, but also a tone reproduction wherein the density changes with a small pattern width.